One of the primary functions of a cardiac monitor is to reliably measure the heart rate of a patient. Conventionally, electrocardiographic electrodes are used to sense the patient's heart rate, and the signal derived from such electrodes is applied to the cardiac monitor. A problem arising from the use of such electrodes is that muscle twitch, or muscle artifact, may generate signals in the electrodes which will cause erroneous heart rate readings. It is possible to avoid such electrode problems by sensing the patient's pulse rate with a pressure transducer connected invasively or a pulse device attached to the finger or ear of the patient. However, random erroneous signals may also be detected by a pressure transducer or pulse sensor as a result, for example, of external pressure applied to the patient's body during the monitoring procedure.
A solution to the problems arising from these two forms of sensing a patient's pulse rate, in the past, was to provide a switching device to enable the user to select either the ECG source or the pressure source for triggering a heart rate meter.
An object of the present invention, however, is to provide an automatic selecting circuit which senses the most periodic or reliable one of the two heart rate sources and which applies that selected source as the input signal to the cardiac monitor.